1. Field of the Invention
This invention relates to an apparatus for reproducing signals recorded in a recording medium. More particularly, it relates to an information reproducing apparatus which can appropriately treat reproduced signals having a fluctuation in low frequency components.
2. Related Art
In an optical disk drive, the shape of a record mark fluctuates for such reasons as the fluctuation of the emission power of a semiconductor laser ascribable to the surroundings of the laser, and the fluctuation of the sensitivity of an optical disk medium ascribable to the surroundings of the medium and the discrepant sensitivities of individual media. In a reproducing or playback operation, the fluctuation of the shape of the record mark incurs even-order distortions, resulting in the disadvantage that the duty factor of a reproduced signal fluctuates. It is necessary to eliminate the distortions to appropriately control the slice level of a wave-shaping circuit (binarizing comparator) for turning the reproduced signal into a binary level. An example of a method for the slice level control is stated in "Signal Processing Technique in Optical Recording" compiled by Kunimaro Tanaka, issued by Kabushiki-Kaisha Torikeppusu in 1989, pp. 116-118.
FIG. 3 of the accompanying drawings illustrates an example of an arrangement of a signal reproducing circuit. A laser beam is focused onto the track of an optical disk that has been subjected to mark length recording. Light thus reflected from the optical disk is used for obtaining a reproduced signal a, which is amplified to a desired amplitude by an amplifier 1. The reproduced signal a undergoes the fluctuation of a low frequency component (hereinbelow, the fluctuation shall be called the "retardation") on account of the fluctuations of the reflectance and double-refractive index of the disk, the deviation of the laser beam from the center of the track, etc. The amplified reproduced signal is passed through a high-pass filter (hereinbelow, abbreviated to "HPF") 2 in order to eliminate the retardation.
Subsequently, in order to demodulate recorded information, the reproduced signal b delivered from the HPF 2 is input to a wave-shaping circuit 3 which includes a reproduced-signal binarization means. On the other hand, the reproduced signal b is also input to an upper or peak envelope detecting circuit 4 and a lower or bottom envelope detecting circuit 5 which detect the peak and bottom envelopes of the input signal b, respectively. The outputs c and d of the respective detectors 4 and 5 have their difference e taken by a difference circuit 6, thereby obtaining a level which is equal to 1/2 of the amplitude of the input signal b of the wave-shaping circuit 3.
The slice level of the input waveform of the wave-shaping circuit 3 is set at the 1/2 level of the amplitude of a signal reproduced from a sync region for phase-locking a demodulating clock signal. The synchronizing signal of the sync region generated by a VFO (variable frequency oscillator) is contained in the headers of a plurality of areas (hereinbelow, termed "sectors") into which the recording medium is divided, and the aforementioned sync region shall be also termed the "VFO part" below.
For the purpose of the above level setting, a sample-and-hold circuit 7 is controlled by a VFO gate f indicative of the VFO part so as to apply the 1/2 level of the VFO part as the slice level g of the wave-shaping circuit 3. The 1/2 level of the VFO part is adopted as the slice level g because the signal duty factor of the waveform of the VFO part is 50 in the mark length recording, so the center level of the waveform becomes the appropriate slice level of the recorded data.
Further, the setting of the slice level g is limited to the VFO part for the following reasons: The VFO part serves as a reference for phase-locking the reproduced signal. Since the part of the reproduced signal except the VFO part has a diverse duty factor and becomes different in the sensitivities of peak envelope detection and bottom envelope detection, it does not serve as the appropriate slice level for all the reproduced signals.
FIG. 4 illustrates the waveforms of the signals at the various parts of the signal reproducing circuit shown in FIG. 3. Hereafter, the circumstances of the signal reproduction shall be explained concerning a case where the signal is recorded on only a part of one track as depicted at a in FIG. 4. Further, for brevity of explanation, it is assumed that the waveform of the VFO part has a signal duty factor of 50, so the center level thereof, being the appropriate slice level for the binarization (for turning the reproduced signal into the binary level), is equal to the mean level between a non-recording level and a DC (direct current) recording level.
The reproduced signal a shown in FIG. 4 is passed through the HPF 2 giving a resultant waveform b in FIG. 4, which is applied to one input of the wave-shaping circuit 3. On the other hand, the outputs of the peak envelope detecting circuit 4 and bottom envelope detecting circuit 5 become as shown by the waveforms c and d in FIG. 4, respectively. The output of the difference circuit 6, which is the difference waveform of the outputs c and d, becomes as shown by the waveform e in FIG. 4. This waveform e is equal to the DC level fluctuation of the reproduced signal which has developed in the HPF 2 at the VFO part (namely, in correspondence with the high level of the VFO gate f shown in FIG. 4). At any other part, however, the waveform e is not necessarily equal to the DC level fluctuation which has developed in the HPF 2. For this reason, the VFO gate f is set at the low level except for at the VFO part, and the output e of the difference circuit 6 is held in correspondence with the final part or trailing edge of the VFO part by the sample-and-hold circuit 7. Thus, the waveform g shown in FIG. 4 is applied to the other input of the wave-shaping circuit 3. Incidentally, a waveform indicated by a broken line at g in FIG. 4 is the waveform b which is one input signal of the wave-shaping circuit 3 as stated above.
In the case as shown in FIG. 4 where the VFO part is shorter than the transient response time of the settling of the DC level fluctuation based on the HPF 2, the slice level of the wave-shaping circuit 3 is held before the DC level fluctuation has settled. In consequence, the slice level deviates from the appropriate value (indicated by a dotted line at g in FIG. 4) to the amount of the DC level fluctuation of the HPF 2 which arises after the holding operation. This incurs the problem that the signal recorded on only part of the track cannot be read correctly.
The problem is not limited to the case where the signal is recorded on only part of the track, but similar problems occur in cases where momentary DC level fluctuations are involved in the reproduced signal. More specifically, the momentary fluctuations arise, for example, i) at the boundary between a preformat part (such as an address signal part) and an additionally written data part (data signal part) in the magneto-optical disk, ii) immediately after the operation of seeking a track, iii) immediately after the reproduction of a major defect, and iv) immediately after recording or erasing a signal. Regarding the item i), a preformat signal (such as an address signal) in the preformat area is recorded in the form of ruggedness (such as grooves or pits) of the recording medium. On the other hand, a data signal which is additionally written in the data area by a user is recorded in the form of a change of the magnetic field of the recording medium. Therefore, detecting methods for the two signals are different, and the DC levels of a reproduced signal respectively obtained from the preformat area and the data area are also different. Meanwhile, regarding the items ii).about.iv), correct signals are not detected immediately after the seek operation, the reproduction of the defect, and the recording or erasing operation. Signals reproduced on these occasions have diverse DC levels. When the correct signals are thereafter reproduced, the DC level fluctuations take place.
Moreover, when the VFO part, being a specific pattern part, is defective, the control means based on the signal of the specific pattern part cannot perform an appropriate control.